Introduction

Automatic Block Signals (Danish: Automatiske Mellembloksignaler)
are found on most multiple track mainlines in Denmark. Most double track
in Denmark is equipped for unidirectional running, with the possiblity
of occational wrong main running. Bidirectional Signaling (Danish:
Vekselblok)
has been the standard on new installations since 1986, but is yet not very
widespread. Since multiple bidirectional tracks are essentially a number
of parallel single tracks, bidirectional installations are considered single
track installations in this document.

Automatic Block Signals are rarely found on single track, due to the
fact that most single track lines in Denmark see short distances between
stations or/and low traffic volumes.

Automatic Block Signal Types

Automatic Block Signals may be divided into two physical and two logical
types. The physical types are a 2-lamp and a 3-lamp type as shown below.
A variation of the 2-lamp signal is a 4-lamp signal found on the "S-Bane"
(the Copenhagen Suburban Network) including two yellow lamps for a cab
signaling aspect.

Logically, Automatic Block Signals are divided into advance signaling
Automatic Block Signals and non advance signaling Automatic Block Signals.
Non advance signaling Automatic Block Signals are found where the next
Main Signal (Entry Signal or Automatic
Block Signal) is preceded by a Distant Signal.
Non advance signaling Automatic Block Signals are marked by a yellow
plate with a black letter "F" on the mast (left).

Non advance signaling Automatic Block Signals are always the 2-lamp
type. Advance signaling Automatic Block Signals on main lines are always
the 3-lamp type. Almost all double track on the S-bane features the 4-lamp
Automatic Block Signal type though one line features a 2-lamp advance signaling
type and yet another features a mixture of advance and non advance signaling
signals. Some installations vary from the rule. Automatic Block Signals
and Exit Signals on the same line are
almost always the same type.

Automatic Block Signal Aspects and Indications

"Stop" (Danish: "Stop")

Stop

X

X

X

X

"Stop and Proceed" (Danish: "Stop og Ryk Frem")

Stop, then proceed to next Main Signal. Maximum speed in the next block
is 60 km/h (40 mph). Next Main Signal must be expected to show "Stop"

X

X

X

"Proceed" (Danish: "Kør")

Proceed at prescribed speed

X

X

X

X

The signal does not convey information about indication on the following
Main Signal (distant signal to follow)

X

Expect next Main Signal to show "Stop" (A following Entry Signal may show
"Proceed" or "Proceed Through" with a permitted speed of less than 75 km/h)

X

X

X

Trains with active Cab Signal: Cab signal will govern

X

X

X

X

"Proceed Through" (Danish: "Kør Igennem")

Proceed at prescribed speed. Next Main Signal shows "Proceed" or "Proceed
Through" (with a permitted speed of at least 75 km/h if next signal is
an Entry Signal)

Automatic Block Signal Usage

Double Track Usage

Main Lines

The by far most common use of Automatic Block Signals on main lines is
as unidirectional doubletrack block signaling, such as it is outlined in
the document about Line Block System type
DSB 1954 . The Automatic Block Signals are the 3-aspect, 3-lamp type:

The Automatic Block Signals are usually approach lit. The green lamps
are dimmed when no train is present in the block approaching the signal.
The "Stop" aspect is never dimmed:

Main and Distant Signals

In a few locations, Automatic Block Signal spacing is too far to make advance
signaling through the previous Automatic Block Signal practical. These
lines instead feature a combination of 2-aspect Automatic Block Signals
and 2-aspect Distant Signals (signals are approach lit):

Older Suburban Lines

A few older Suburban lines around Copenhagen feature a variation of the
main line Automatic Block Signaling with 2-lamp, 3-aspect Automatic Block
Signals. The "Proceed Through" aspect is shown as a flashing green lamp
instead of the double green:

Otherwise the functions of this system are identical to the main line
system:

Copenhagen Suburban Lines

Most Copenhagen Suburban lines feature line block and station interlocking
systems utilizing cab signals and a speed supervision system. This so called
HKT-system
(Danish: HastighedsKontrol og Togstop, SpeedCheck and Trainstop)
permits trains with active cab signals to run without the need for lineside
signals. Lineside signals are provided on such lines, though only as a
fall-back measure for trains with failed cab signals.

Trains are permitted to follow each other in blocks sections of so called
HKT-sections
(Danish: HKT-afsnit), separated by rectangular red and white signs.
The Automatic Block Signals are approach lit and normally show the special
"Conditional Stop" aspect, permitting trains with active cab signal to
proceed:

The Automatic Block Signals switch to "Stop" when the HKT-section immediately
behind the signal is occupied:

As soon as the first HKT-section behind the signal clears, the Automatic
Block Signal switches to "Conditional Stop" again, permitting the next
train to proceed on cab signal indication. The Automatic Block Signal displays
the lit "Conditional Stop" aspect until the train passes the next lineside
signal:

Trains with failed cab signal (or without cab signal) are governed by
ordinary signal aspects. The line block system receives a command to switch
to lineside signaling from the neighboring station interlocking. The Automatic
Block Signals are then prevented from showing "Conditional Stop" for the
train in question, remaining at "Stop" until an absolute block (to the
next lineside signal) can be established:

The Automatic Block Signals automatically revert to normal after the
passage of the train:

Once switched to lineside signaling, the line cannot be switched back
to cab signal only running. So even if, say, trains are swapped, the first
train will run by lineside signals. Since the wayside cab signal system
is always active, however, a train with working cab signal will always
be supervised by the HKT-system.

The Copenhagen Suburban automatic line block system (called type DSB
1969) features facilities for wrong main running similar to the features
of the Line Block System type DSB 1954
. After wrong main running the type DSB 1969 line block system switches
to the lineside signaling mode for the first train. Since the line is completely
clear in this situation, and the wayside cab signaling is active, there
is no loss of traffic capacity or safety.

Single Track Usage

Older Mainline systems

A few older lines have a single set of Automatic Block Signals and Distant
Signals between two stations. The green lamp of the Automatic Block Signal
and the Distant Signal are approach lit:

Multiple Track Bidirectional running

New mainline installations are built for bidirectional running. Each track
has Automatic Block Signal for both directions. The use of signal aspects
follow the practises for mainline systems, though approach lighting is
no longer used.

The line block has a set direction of travel, the direction being changed
when a station sets an exit route for a train opposing the already set
direction. Signals in the direction of travel show lit proceed aspects,
opposite direction has all signals at "Stop":

On later bidirectional installations, all signals normally show "Stop".
The block system still has a set direction, but the signals clear only
when a train is approaching. The Automatic Block Signals then remain at
"Stop" until the next train approaches.

Keeping signals at "Stop" avoids resetting signals to "Stop" when reversing
traffic on a track. The "Stop"-setting of the Automatic Block Signals
has often led to an emergency braking when a locomitive engineer traveling
along one of a numbr of parallel tracks is uncertain which of many signals
actually went to "Stop".

"Stop and Proceed"

Due to technical failures a Block Signal sometimes stays at "Stop" even
though the train behind the signal has entered the following block. In
this situation the line may be released for "Stop and Proceed", meaning
that all Block Signals showing red to an apparently free block section
switches to "Stop and Proceed". The release is
valid for all signals on the track:

Most multiple track line block systems in Denmark feature the "Stop
and Proceed" aspect, an important exception being the Copenhagen Suburban
systems for cab signaling.